A typical conventional liquid crystal display (LCD) device is a hold-type display device (also referred to as “sample-and-hold-type” display device), in which pixels of an image are held (or kept constant) for the duration of a frame time period. In one example, the hold time can be a vertical period of 16.7 ms (milliseconds). Recently, with large scale LCD panels used with personal computers (PCs) or televisions (TVs), increased emphasis has been placed on improving the display quality of a motion picture by the LCD panel. Blurring of displayed images of a motion picture is due to a blurred edge profile occurring when a frame is being switched. There are two causes of a blurred motion picture. One is the response time of the liquid crystal, and the other is the active matrix liquid crystal driving method that involves use of TFTs (thin-film transistors).
In general, the response time of the liquid crystal is important to the motion picture display quality. The switching time of the one frame is typically about 16.7 ms in LCD panels used in TVs. Therefore, whether or not a liquid crystal layer used in a TV LCD panel can achieve a response time of 16.7 ms or less is a factor in determining whether blurring will occur.
However, nevertheless, even if the response time of the liquid crystal layer is 0 ms, blurring may still occur because a typical LCD device displays an image in the hold-type manner.
FIG. 1 is a graph showing a motion picture response time versus a previous gray level and a current gray level. As shown in FIG. 1, the pixels in area A are the ones switched from a low gray-scale level in the previous frame to a high gray-scale level in the current frame, and the pixels in area B are the ones switched from a high gray-scale level in the previous frame to a low gray-scale level in the current frame. The response time of each pixel in either the area A or the area B is longer, so the quality of the motion picture in areas A and B is relatively poor and the blur phenomenon may occur.
Conventionally, the blur phenomenon of a motion picture caused by a hold time may be improved by using dynamic scanning backlight, in which the backlight source is turned on and off according to the frame frequency and phase, and by using an intermittent display method with black signal insertion. Generally, the intermittent display method with black signal insertion involves synthesizing a gray-scale level by successively displaying one high gray-scale level and one low gray-scale level. Thus, a gray-scale lookup table (or High-Low Lookup Table or LUT) may be disposed in the display system. For a given input gray-scale level (that is to be displayed), the lookup table is accessed to obtain the corresponding high gray-scale level and the corresponding low gray-scale level, which are used to successively drive a pixel in a single frame.
FIG. 2 is a graph showing four different areas depending on a previous gray level versus a current gray-scale level. As shown in FIG. 2, when the gray-scale level of a pixel is represented with 8 bits, the range of gray-scale level is from 0 to 255. The gray-scale level of a given gray point (GP) may be defined by successively displaying a full white gray-scale level (255) and a full black gray-scale level (0), so that the gray-scale level of the point (GP) can be synthesized and displayed in a dynamically stable condition and determined according to the property of the liquid crystal. In other words, voltages corresponding to the gray-scale levels 0 and 255 may be input to the display panel using two pulse signals so that the gray-scale level of the point GP may be obtained.
As shown in FIG. 2, the previous frame has a gray point GP for a particular pixel, and the current frame has another gray point GP for the particular pixel. In the chart of FIG. 2, the gray point GP of the previous frame is associated with a horizontal line, and the graph point GP of the current frame is associated with a vertical line. The horizontal and vertical lines that intersect the two gray points define four areas: a first area, a second area, a third area and a fourth area. If a pixel with the low gray-scale level in the previous frame is switched to a high gray-scale level in the current frame (as in the second area), or the pixel with the high gray-scale level in the previous frame is switched to a low gray-scale level in the current frame (as in the third area), the gray-scale level variation of the pixel in either the second area or the third area crosses the lines in the chart corresponding to the gray points GP. The quality of the motion picture of pixels that exhibit such gray-scale variations of FIG. 2 can be poor.
FIG. 3 is a timing diagram showing timings for displaying one gray-scale pulse signal in a synthesized manner by sucessively displaying the high gray-scale level and the low gray-scale level, wherein m is the number of scan lines in a display device. Each scan line is turned on at least twice (with two corresponding pulses 100A and 100B) in one frame time, and the time interval between the two successive turn-on pulses 100A and 100B is defined as a refresh time. Each pulse in FIG. 3 is a voltage signal of a scan line. A signal corresponding to a low gray-scale level is provided on a scan line during a first refresh time (pulse 100A), while a signal corresponding to a high gray-scale level is provided on the scan line during a second refresh time (pulse 100B). Alternatively, a signal corresponding to a high gray-scale level may be provided in the scan line during the first refresh time, while a signal corresponding to a low gray-scale level is provided on the scan line during the second refresh time. Additionally, scan lines may be turned on in the following order. The first scan line is first turned on (pulses 100A and 100B). Next, the (n+1)th scan line is turned on (pulses 100C and 100D). Then, the second scan line is turned on (pulses 100E and 100F). Next, the (n+2)th scan line is turned on (100G and 100H), and so forth, until all the scan lines have been turned on. Thereafter, for the next frame, the scan lines are again turned on according to the above-mentioned order. Thus, it is possible to derive a refresh time as equal to (m−n/m)×one frame time, where the refresh time can be dynamically changed by adjusting the value of n.
FIGS. 4A to 4C are schematic illustrations showing that the gray point position (gray-scale level) is changed by controlling the time for successively displaying the high gray-scale level and the low gray-scale level from the same gray-scale lookup table (High-Low LUT), as conventionally done. That is, the gray point position (gray-scale level) may be changed by controlling the widths of the applied pulse signals by reference to the high gray-scale level lookup table (High LUT) and the low gray-scale level lookup table (Low LUT).
As shown in FIG. 4A, the time for displaying the high gray-scale level is controlled to be shorter (see the I area) while the time of displaying the low gray-scale level is controlled to be longer (see the II area). In other words, the driving time corresponding to the high LUT is controlled to be shorter, and the driving time corresponding to the low LUT is controlled to be longer. Thus, the gray point position (gray-scale level) may be adjusted in the direction toward the lower gray-scale level. As shown in FIG. 4B, the time of displaying the high gray-scale level (see the I area) and the time of displaying the low gray-scale level are controlled to be the same. In other words, the driving time corresponding to the high LUT and the driving time corresponding to the low LUT are controlled to be the same. As shown in FIG. 4C, the time of displaying the high gray-scale level is controlled to be longer (see the I area) and the time of displaying the low gray-scale level is controlled to be shorter (see the II area). In other words, the driving time corresponding to the high LUT is controlled to be longer, and the driving time corresponding to the low LUT is controlled to be shorter. Thus, the gray point position (gray-scale level) may be adjusted in the direction toward the higher gray-scale level.